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Scientists who have sequenced the genome of the sea urchin say these brainless and limbless invertebrates are surprisingly similar to humans.

They found that the California purple sea urchin (Strongylocentrotus purpuratus) genome has 23,300 genes.

And it shares 7077 of them with humans.

The genetic ties are far closer than scientists expect and make the sea urchin a closer genetic cousin of humans than the worm or fruit fly, according to the study in today's issue of the journal Science.

"Nobody would've predicted that sea urchins have such a robust gene set for visual perception," says Gary Wessel, a Brown University biology professor and member of the Sea Urchin Genome Sequencing Consortium.

"I've been looking at these organisms for 31 years, and now I know they were looking back at me."

Among other surprises from the project were that researchers found sea urchins have the most sophisticated innate immune system of any animal studied to date.

They say this may be one reason they live 100 years or more.

Sea urchins also carry genes associated with human diseases such as muscular dystrophy and Huntington's.

The creatures also have genes associated with taste, smell, hearing and balance, the study found.